Communication systems are known to include a communication resource controller, a plurality of communication units, and communication resources that are transceived via base stations or repeaters. In such communication systems, the communication units communicate with each other via the communication resources, where a communication resource may be an RF channel, a pair of RF channels, a TDM slot, or any medium for carrying RF signals. To initiate a communication, a communication unit transmits a communication request to the communication resource controller via a communication resource dedicated for control information (control channel). The communication request includes a request for a particular service, such as allocation of a communication resource and the identity of a target communication unit, or units. For example, the request may be for a group call which identifies all the communication units in the same group as the initiating communication unit.
The communication request is received by the control channel repeater and routed to the communication resource controller. Upon receipt of the request, the communication resource controller determines whether the communication unit is authorized to access the system and, if so, grants the request. Having granted the request, the communication resource controller allocates a communication resource, and the associated base station, to the initiating communication unit and the target communication units, and then transmits a communication resource allocation message on the control channel. All communication units within the coverage area of the control channel antenna receives the allocation message. However, only the communication units identified in the message, i.e., the initiating and target communication unit, will access the allocated communication resource and subsequently partake in the communication.
This method of communication resource allocation works very well in many trunking communication system applications, however, because there is a limited number of communication resources, this method has its limitations. For example, assume that a particular communication group is involved in a group call which is being supported on a communication resource. Further assume that all the members of the communication group call are within close proximity. In this instance, if the communication units could transceive information directly to one another, a system communication resource could be reserved for another system call.
Motorola provides a dual mode communication unit, i.e., one that operates on either a system communication resource or in a direct mode. This unit allows the user to exclusively operate his or her communication unit in either the direct mode or the system mode. To switch from one mode to the other, this unit must be manually adjusted, thus it does not resolve the above mentioned problem.
Therefore, a need exists for an automatic method of changing communication modes from a communication system supported mode to a direct mode of communication.